Devices of the above-specified type are described, in particular, in U.S. Pat. Nos. 3,630,078 and 3,934,467. Both of these documents describe helical rotor flowmeters, in which the measuring means are constituted by a spinner immersed in the fluid stream and rotating at a rate which is a function of the flow rate. A drawback associated with such devices is that they include a large number of mechanical moving parts which are fragile and difficult to construct. Another drawback is that the space occupied by the spinner makes it difficult to insert in a fluid stream, particularly when the spinner must be passed through zones of small cross section: proposals have even been made for spinners having retractable blades, thereby further complicating the flowmeter as a whole. Finally, whenever a spinner is inserted in a stream, an obstruction is set up and under some circumstances this may be undesirable.
The aim of the present invention is to provide a nonintrusive flowmeter, and in particular a flowmeter which does not include a spinner, but which is simple in structure and which includes a minimum of moving parts.